Re: Pileups Reported on Snow-Covered Roads (

From: Stephen D. Williams (
Date: Thu Feb 22 2001 - 15:14:52 PST

Chuck Murcko wrote:
> And BTW, WTF is "black ice"? This term seems to have become popular last decade and I haven't noticed a new phase of water or anything, just the same stuff that's always been frozen on the roads. Or is it some kind of doofus mental aid?

It means a clear road covered by clear ice. The road loses it's
specularity and becomes much darker, hence 'black ice'. Imagine
starting with no snow, having a rain or very heavy mist, then the
temperature suddenly drops with no more precipitation. The standing
water can create a very smooth, hard ice layer that can remain wet.
Doesn't happen often, although I was very lucky to be going slowly in
Northern Ohio once as the temperature dropped from 34 to 28. The wet
road changed suddenly and was so slippery I could barely stand.

Specularity is the (typically) bell-curve random reflection of light.
Mirrors have zero specularity. Most surfaces have some specularity
which allows you to see them with light coming from non-incident angles
since some will get scattered in every direction.

In warmer states, like Virginia, the temperature tends to bounce over
and under freezing which gives more opportunities for this.

> Chuck Murcko
> Topsail Group

Stephen D. Williams         Insta, Inc./Jabber.Com, Inc./CCI
43392 Wayside Cir,Ashburn,VA 20147-4622 703-724-0118W 703-995-0407Fax 

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